The hero of the Welsh folktale Einion and Olwen, in which he travels to the Otherworld to marry Olwen, the daughter of the chief giant Yspaddaden. They had a child whom they named Taliesin.
Einion would appear to be a localised direct replacement for Culhwch, the hero of the Mabinogion story of Culhwch and Olwen, who is usually connected with the quest to marry Olwen. However, as Culhwch and Olwen is generally considered an Arthurian story, the reverse may be true, with Culhwch replacing Einion, whose story would then be the earlier.